How do I train my cat to start using a litter box again?

I recently moved into my girlfriends house with my female cat, Guinness. My girlfriend already had two female cats living with her. Guinness is the oldest and biggest out of the three and still after months does not get along with her cats (fighting, hissing, etc... at every chance). The most stressfull thing is she refuses to use any of the three litter boxes I have set up for them. I clean them out daily and even tried to strategically place them throughout the basement for encouragement.... no use. I still come home everyday to piles on the floor around the litter boxes.

Status

Cast your vote for which answer you think is best!

Answers

Are all the cats spayed? If not this should be done asap. Also, you might want to try putting a box just for Guinness in another room separate from your girlfriend's cat's boxes. If there are social issies, Guinness will not want to "go" where they do. Stress is one of the biggest reasons cats stop using the box. You may want to check with her vet and ask for a short course of kitty prozac for her.